Currently, top performance shrink films used for demanding collation applications are manufactured by blown film process and typically comprise 3 layers. The two skin layers are usually based on blends of majority of Linear Low Density Polyethylene (LLDPE) (often metallocene single-site catalyzed Linear Low Density Polyethylene (m-LLDPE)) with Low Density Polyethylene (LDPE). The central layer typically contains fractional melt Melt index Low Density Polyethylene (MI LDPE) occasionally blended with High Density Polyethylene (HDPE). Increasing demand for higher modulus films that would allow for down gauging of today's typical 45 micron collation shrink films down to 35 microns or less has arisen. Such a significant increase in modulus cannot be addressed by known resins combined with existing formulations and existing 3 layer coex technology. While converters currently attempt to increase modulus by increasing the content of stiff resins (mainly HDPE and occasionally PP), there are limitations to this approach in terms of minimizing resin usage and detrimentally effecting other properties.
However, it has been discovered that 5-layer machinery which has become more prevalent in the industry together with innovative film formulations may be used to achieve films to meet these sustainability goals. Accordingly, one aspect of the present invention is a multilayer film which can be downguaged to 40 microns or less and yet still has good stiffness and adequate shrink properties.
Thus, a first aspect of the present invention is a multilayer film comprising four or more discrete layers. The film comprises a first and a second external layer, at least one stiffening layer and at least one shrink layer. Each external layer independently comprises from 60 to 100% by weight of the external layer of a linear low density polyethylene resin characterized by having a melt index in the range of 0.3 to 4 g/10 min and a density in the range of from 0.917 to 0.950 g/cm3, and a polydispersity of from 3 to 9.
The required stiffening layer comprises from 60 to 100% by weight of the stiffening layer of a stiffening resin selected from the group consisting of 1) a polypropylene resin, wherein said polypropylene resin is selected from the group consisting of homopolymer polypropylene and a propylene random copolymer derived from more than 90% by weight propylene and up to 10% by weight ethylene or one or more alpha olefins having 4 to 10 carbon atoms, wherein the polypropylene resin is characterized by having a melt flow rate in the range of 0.1 to 1 g/10 min; and 2) a high density polyethylene resin characterized by having a melt index in the range of 0.25 to 4 g/10 min and a density in the range of from 0.935 to 0.975 g/cm3; and 3) blends thereof.
The required shrink later comprises from 50 to 100% by weight of the shrink layer of a high pressure low density polyethylene resin characterized by having a melt index in the range of 0.2 to 1 g/10 min and a density in the range of from 0.917 to 0.935 g/cm3.
The films of this aspect of the invention can be characterized by having a gloss of at least 62% at 45 degrees, together with a 2% secant tensile modulus greater than 400 MPa in the machine direction, the cross direction or both the machine direction and the cross direction. Preferably such films have a total thickness of 40 microns or less.